The present invention relates to an expandable stent.
Stents are generally known. Indeed, the term xe2x80x9cstentxe2x80x9d has been used interchangeably with terms such as xe2x80x9cintraluminal vascular graftxe2x80x9d and xe2x80x9cexpansible prosthesisxe2x80x9d. As used throughout this specification the term xe2x80x9cstentxe2x80x9d is intended to have a broad meaning and encompasses any expandable prosthetic device for implantation in a body passageway (e.g. a lumen or artery).
In the past six to eight years, the use of stents has attracted an increasing amount of attention due the potential of these devices to be used, in certain cases, as an alternative to surgery. Generally, a stent is used to obtain and maintain the patency of the body passageway while maintaining the integrity of the passageway. As used in this specification, the term xe2x80x9cbody passagewayxe2x80x9d is intended to have a broad meaning and encompasses any duct (e.g. natural or iatrogenic) within the human body and can include a member selected from the group comprising: blood vessels, respiratory ducts, gastrointestinal ducts and the like.
Initial stents were self-expanding, spring-like devices which were inserted in the body passageway in a contracted state. When released, the stent would automatically expand and increase to a final diameter dependent on the size of the stent and the elasticity of the body passageway. An example of such a stent is known in the art as the Wallstent(trademark).
The self-expanding stents were found by some investigators to be deficient since, when deployed, they could place undue, permanent stress on the walls of the body passageway. Further, upon expansion, the stent would shorten in length in an unpredictable fashion thereby reducing the reliability of the stent. This led to the development of various stents which were controllably expandable at the target body passageway so that only sufficient force to maintain the patency of the body passageway was applied in expanding the stent.
Generally, in these later systems, a stent, in association with a balloon, is delivered to the target area of the body passageway by a catheter system. Once the stent has been properly located (for example, for intravascular implantation the target area of the vessel can be filled with a contrast medium to facilitate visualization during fluoroscopy), the balloon is expanded thereby expanding the stent by plastic deformation so that the latter is urged in place against the body passageway. As indicated above, the amount of force applied is at least that necessary to maintain the patency of the body passageway. At this point, the balloon is deflated and withdrawn within the catheter, and subsequently removed. Ideally, the stent will remain in place and maintain the target area of the body passageway substantially free of blockage (or narrowing).
A stent which has gained some notoriety in the art is known as the Palmaz-Schatz(trademark) Balloon Expandable Stent (hereinafter referred to as xe2x80x9cthe Palmaz-Schatz stentxe2x80x9d). This stent is discussed in a number of patents including U.S. Pat. Nos. 4,733,665, 4,739,762, 5,102,417 and 5,316,023, the contents of each of which are hereby incorporated by reference.
Another stent which has gained some notoriety in the art is known as the Gianturco-Roubin Flex-Stent(trademark) (hereinafter referred to as xe2x80x9cthe Gianturco-Roubin stentxe2x80x9d). This stent is discussed in a number of patents, including U.S. Pat. Nos. 4,800,882, 4,907,336 and 5,041,126, the contents of each of which are hereby incorporated by reference.
Other types of stents are disclosed in the following patents:
U.S. Pat. No. 5,035,706 (Gianturco et al.),
U.S. Pat. No. 5,037,392 (Hillstead),
U.S. Pat. No. 5,147,385 (Beck et al.),
U.S. Pat. No. 5,282,824 (Gianturco),
Canadian patent 1,239,755 (Wallsten), and
Canadian patent 1,245,527 (Gianturco et al.),
the contents of each of which are hereby incorporated by reference.
While these prior art stents have achieved a varying degree of success, the art is constantly in need of new stents having improved flexibility and stability while being able to be readily implanted with little or no trauma to the target lumen.
In our Canadian patent application number 2,134,997 (Penn et al.), the contents of which are hereby incorporated by reference, there is described an improved expandable stent. The stent comprises a tubular wall disposed between the proximal end and the distal end. The tubular wall has a longitudinal axis and a porous surface defined by a plurality intersecting members arranged to define a first repeating pattern. The first repeating pattern comprises a polygon having a pair of side walls substantially parallel to the longitudinal axis. A first concave-shaped wall and a second convex-shaped wall connect the side walls. The first wall and the second wall are equidistant along an axis which is parallel to the longitudinal axis. The stent is expandable from a first, contracted position to a second, expanded position upon the application of a radially outward force exerted on the stent.
As disclosed in the ""997 application, the first repeating pattern can be implemented in, inter alia, a mono-tubular expandable stent and a bifurcated expandable stent.
While the stent disclosed in the ""997 application is an advance in the art, in certain cases, a significant force is required to achieve expansion in the target body passageway. Further, implantation of the stent disclosed in the ""997 application can be difficult in certain situations where the unexpanded stent must travel through a significantly curved pathway to the target body passageway.
Accordingly, it would be desirable to have an improved stent which overcomes these disadvantages. It would be further desirable if the improved stent could be readily adapted, inter alia, to mono-tubular expandable stents and bifurcated expandable stents. The latter type of stents would be useful in treating aneurysms, blockages and other ailments. It would also be desirable if such a stent was relatively easy to implant. It would be further desirable if such a stent were capable of being uniformly expanded at relatively low pressure while obviating or mitigating longitudinal shrinkage thereof. It would be further desirable if such a stent were not susceptible to asymmetric internal coverage of the body passageway, a problem associated with xe2x80x9ccoilxe2x80x9d-type stentsxe2x80x94see, for example, U.S. Pat. No. 5,282,824 (Gianturco). It would be further desirable if such a stent was not susceptible to movement along the longitudinal axis of the body passageway during or after implantation. It would be further desirable if such a stent was characterized by a desirable balance of lateral flexibility in the unexpanded state and radial rigidity in the expanded state.
It is an object of the present invention to provide a novel expandable stent which obviates or mitigates at least one of the above-mentioned disadvantages of the prior art.
Accordingly, in one of its aspects, the present invention provides an expandable stent comprising a proximal end and a distal end in communication with one another, a tubular wall disposed between the proximal end and the distal end, the tubular wall having a longitudinal axis and a porous surface defined by a plurality of intersecting members comprising a series of longitudinal struts disposed substantially parallel to the longitudinal axis of the stent, each of the longitudinal struts comprising flexure means for substantially complementary extension and compression of a diametrically opposed pair of the longitudinal struts upon flexure of the stent, the stent being expandable from a first, contracted position to a second, expanded position upon the application of a radially outward force on the stent.
Thus, in this aspect of the present invention, we have now discovered that the use of flexure means in the series of longitudinal struts leads to a very desirable balance of lateral flexibility of the unexpanded stent and radial rigidity of the expanded stent. Practically, the flexure means confers lateral flexibility to the unexpanded stent by allowing diametrically opposed pairs of the longitudinal struts to undergo substantially complementary extension and compression. If one considers a stent in a flexed state, a first longitudinal strut disposed at the tangent of the bend (i.e. in two dimensions) will expand in response to the bending moment. In contrast, a second longitudinal strut disposed diametrically opposite (this can mean above, below or in the same radial plane as) the first longitudinal strut will compress in response to the bending bend moment. Generally, the degree of extension and compression will be substantially complementary. In other words, in most cases, the first longitudinal strut will expand and lengthen a first distance and the second longitudinal strut will compress and shorten a second distance. Preferably, the first distance is greater than the second distance and most preferably, the sum of the first distance and the second distance is substantially equal to the sum of the original lengths of the first longitudinal strut and the second longitudinal strut.
The specific shape of the flexure means disposed in the longitudinal strut is not particularly restricted provided that it confers lateral flexibility to the unexpanded stent by allowing diametrically opposed pairs of the longitudinal struts to undergo substantially complementary extension and compression. The term xe2x80x9cdiametrically opposed pairs of the longitudinal strutsxe2x80x9d, as used in this specification, is intended to have a broad meaning. Thus, the xe2x80x9cpairxe2x80x9d can include opposed struts in the same horizontal plane (i.e. the same ring of polygons) or in different horizontal planes (e.g. one strut in a first ring of polygons and the other diametrically opposed strut in a second ring of polygons above or below the first ring). Preferably, the flexure means comprises at least one lateral section disposed in the longitudinal strut, more preferably at least a first lateral section and a second lateral section disposed in the longitudinal strut. By xe2x80x9clateral sectionxe2x80x9d is meant a section of the longitudinal strut which is bowed in or out of (i.e. radially from) the strut. The apex of the lateral section may be pointed, rounded or substantially flat. When the flexure means comprises a first lateral section and a second lateral section, the two sections may be symmetric or asymmetric (in the case of asymmetric this includes two sections of the same shape but different size and two sections of different and size). Further, when the flexure means comprises a first lateral section and a section lateral section, the sections may be bowed in the same or opposite direction.
A particularly preferred embodiment of the flexure means comprises a sinusoidal or S-shaped section (various examples of such a section are illustrated herein and discussed below). Preferably, the sinusoidal or S-shaped section is adjacent the second apex of the polygon and the remaining portion of the strut is substantially straight. This feature improves the lateral flexibility of the stent thereby facilitating implantation thereof and may further mitigate longitudinal shortening of the stent upon expansion.
In another preferred embodiment, at least one, more preferably both, of the side walls (i.e. longitudinal struts) of the polygon comprises the sinusoidal or S-shaped section. Preferably, the sinusoidal or S-shaped section is disposed at an end of the side wall. This feature improves the lateral flexibility of the stent thereby facilitating implantation thereof and may further mitigate longitudinal shortening of the stent upon expansion.
When a sinusoidal or S-shaped portion is disposed in the side walls and/or the strut connecting the first apex and the second apex (if present), the precise shape of the portion is not particularly restricted and generally takes the form of an xe2x80x9cSxe2x80x9d. Thus, the sinusoidal or S-shaped portion may be comprised of a pair of joined curved sections wherein each curved section has an arc of about 180xc2x0xe2x80x94i.e. this is illustrated in FIG. 8 of the present application. The term xe2x80x9carcxe2x80x9d denotes the angle from one end of the curved section to the other about the radical point of the curved section. Alternatively, the sinusoidal or S-shaped portion may be comprised of a pair of joined curved sections wherein each curved section has an arc of greater than 180xc2x0xe2x80x94this is illustrated in FIG. 9 of the present application. Further the pair of joined curved sections can be of the same size (this is illustrated in FIGS. 8 and 9 of the present application) or of differing size (this is illustrated in FIG. 10 of the present application), the latter being the most preferred embodiment.
Preferably, the series of longitudinal struts containing the flexure means comprises all substantially longitudinal struts comprised in the plurality of intersecting members making up the porous surface of the stent.
Preferably, for this aspect of the present invention, the intersecting members are arranged to define a first repeating pattern comprised of a polygon having a pair of side walls substantially parallel to the longitudinal axis (i.e. a pair of the above-mentioned longitudinal struts comprising flexure means), a concave-shaped first wall having a first apex and a convex-shaped second wall having a second apex connecting the side walls. As used throughout this specification, the terms xe2x80x9cconcave-shapedxe2x80x9d and xe2x80x9cconvex-shapedxe2x80x9d are intended to have a broad meaning and a shape having apex. Thus, the first wall has a first apex and the second wall has a second apex. Thus, the first apex (i.e. of the concave-shaped first wall) is directed into the polygon whereas the second apex (i.e. of the convex-shaped second wall) is directed away from the polygon.
In another of its aspects, the present invention provides an expandable stent comprising a proximal end and a distal end in communication with one another, a tubular wall disposed between the proximal end and the distal end, the tubular wall having a longitudinal axis and a porous surface defined by a plurality intersecting members arranged to define a first repeating pattern comprised of a polygon having a pair of side walls substantially parallel to the longitudinal axis, a concave-shaped first wall having a first apex and a convex-shaped second wall having a second apex, the first wall and the second wall connecting the side walls, at least one of the first apex and the second apex being substantially flat, the stent being expandable from a first, contracted position to a second, expanded position upon the application of a radially outward force on the stent.
In this aspect of the invention, it is has been discovered that the use of such a first repeating pattern (including at least one of the first apex and second apex being substantially flat), with or without the flexure means present in the side walls of the polygon in the first repeating pattern, results in an improved stent. The advantages associated with the use of such a such a first repeating pattern include the following:
1. the force required to expand the stent is substantially reduced;
2. the stent is subjected to less traumatic stress during expansion;
3. plastic deformation of the stent during expansion is facilitated;
4. construction of the stent is facilitated; and
5. upon expansion of the stent, warpage of the first apex and the second apex is obviated or mitigated.
The provision of at least one of the first apex and the second apex being substantially flat usually results in the apex of the concave-shaped first wall and/or the convex-shaped second wall having a pair of shoulders. Preferably, these shoulders are rounded. The provision of such round shoulders results in the following additional advantages:
6. mitigation of potential trauma to the target body passageway from: (i) endoluminal contents within the passageway, and (ii) the contours of the passageway;
7. the resulting expanded stent is more stream-lined and flow-directed which mitigates potential trauma to the target body passageway;
8. further reduction in the force required to expand the stent;
9. an improved stent expansion ratio is achieved (i.e. ratio of expanded stent diameter at maximum expansion to unexpanded stent diameter);
10. upon expansion of the stent, the concave-shaped first wall and the convex-shaped second wall are in a substantially orthogonal relationship to the longitudinal axis thereby improving the rigidity of the stent (this is very important to mitigate the occurrence of stent recoil); and
11. the pattern of the expanded stent improves the rheology of fluid flow in the body passageway.
When the stent of the present invention includes the above-mentioned first repeating pattern, it is preferred to provide a connecting strut between the first apex and the second apex. Generally, the connecting strut will be substantially longitudinal (i.e. it will be parallel to the longitudinal axis of the stent). This feature mitigates lifting of the shoulders referred to above as the stent is flexed, for example, when passing the stent through a curved body passageway. The result of this is that potential trauma to the body passageway is mitigated since scraping of the body passageway by the shoulders is mitigated.
In a preferred embodiment, the connecting strut is curved with respect to the longitudinal axis (this is described and illustrated hereinbelow). Preferably, the strut is sufficiently curved to have a length of up to about 35%, more preferably up to about 15%, even more preferably in the range of from about 2% to about 8%, most preferably in the range of from about 3% to about 7%, greater than the distance between the first apex and the second apex. This feature improves the lateral flexibility of the stent thereby facilitating implantation thereof. In some cases, the curvature may be designed to comprise the flexure means discussed above. In other words, the shape of the curvature may be designed substantially complementary extension and compression of the connecting strut upon flexure of the stent.
Yet another preferred feature of the stent of the present invention is the provision of one or both of the side walls of the polygon of the repeating pattern being curved. Preferably, both side walls are curved. More preferably the curvature serves as flexure means as described above. Ideally, the curved side wall has length of up to about 35%, more preferably up to about 15%, even more preferably in the range of from about 2% to about 8%, most preferably in the range of from about 3% to about 7%, greater than the distance between the termini of the concave-shaped first wall and the concave-shaped second wall. This feature improves the lateral flexibility of the strut thereby facilitating implantation thereof.
Preferably, both the strut and the side walls are curved. More preferably, each of the curved members are of substantially the same length.
Yet another preferred feature of the stent of the present invention is, in addition to the strut and side walls of the polygon being curved, the provision of all longitudinal walls of the polygon of the repeating pattern being curved. Thus, in this embodiment of the invention, the concave-shaped first wall comprises a pair of curved first apex walls connecting the first apex and the side walls of the polygon, and the convex-shaped second wall comprises a pair of curved second apex walls connecting the second apex and the side walls of the polygon. Again, in some cases, the curvature may be designed to comprise the flexure means discussed above. Ideally, the curved first apex walls and the curved second apex walls each have a length of up to about 35%, more preferably up to about 15%, even more preferably in the range of from about 2% to about 8%, most preferably in the range of from about 3% to about 7%, greater than the straight (i.e. non-curved) distance between the first apex and the side walls, and the second apex and the side walls, respectively. In this embodiment, it is further preferred to have substantially all adjacent curved walls in an annular section of the repeating pattern (i.e. of the struts, first apex wall, second apex wall and side walls) are substantially equidistant from one another. This preferred feature of the stent of the present invention even further enhances the lateral flexibility of the stent thereby further facilitating implantation thereof.
Yet another preferred feature of the stent of the present invention is provision of a porous surface comprising multiple designs. Specifically, in certain cases, it may be desirable to design the stent to varying degrees of relative flexibility and rigidity along the length thereof. Thus, the relatively flexible portion(s) of such a stent would facilitate delivery of the stent to a target body passageway through a relatively tortuous route, while the relatively rigid portion(s) of the stent serves facilitate maintaining the patency of the body passageway. As will be discussed in more detail hereinbelow, this may be achieved by varying the repeating pattern design along the longitudinal length of the stent.
An aspect of the present invention relates to the provision of an expandable bifurcated stent. As used throughout this specification, the term xe2x80x9cbifurcated stentxe2x80x9d is intended to have a broad meaning and encompasses any stent having a primary passageway to which is connected at least two secondary passageways. Thus, trifurcated stents are encompassed herein. Further, one of the secondary passageways can be a continuation of the primary passageway with the result that the other secondary passageway is essentially a side branch to the primary passageway.
The stent of the present invention (bifurcated or mono-tubular) can further comprise coating material thereon. The coating material can be disposed continuously or discontinuously on the surface of the stent. Further, the coating may be disposed on the interior and/or the exterior surface(s) of the stent. The coating material can be one or more of a biologically inert material (e.g. to reduce the thrombogenicity of the stent), a medicinal composition which leaches into the wall of the body passageway after implantation (e.g. to provide anticoagulant action, to deliver a pharmaceutical to the body passageway and the like) and the like.
The stent is preferably provided with a biocompatible containing, in order of minimize adverse interaction with the walls of the body vessel and/or with the liquid, usually blood, flowing through the vessel. The coating is preferably a polymeric material, which is generally provided by applying to the stent a solution or dispersion of preformed polymer in a solvent and removing the solvent. Non-polymeric coating material may alternatively be used. Suitable coating materials, for instance polymers, may be polytetraflouroethylene or silicone rubbers, or polyurethanes which are known to be biocompatible. Preferably however the polymer has zwitterionic pendant groups, generally ammonium phosphate ester groups, for instance phosphoryl choline groups or analogues thereof. Examples of suitable polymers are described in International application number WO-A-93/16479 and WO-A-93/15775. Polymers described in those specifications are hemo-compatible as well as generally biocompatible and, in addition, are lubricious. It is important to ensure that the surfaces of the stent are completely coated in order to minimize unfavourable interactions, for instance with blood, which might lead to thrombosis.
This good coating can be achieved by suitable selection of coating conditions, such as coating solution viscosity, coating technique and/or solvent removal step.
In another embodiment of the invention, the stent may be joined to a polymer material. Specifically, a polymer material may be extruded onto the stent in such a manner that it envelops at least a portion of the stent. This technique may be used to join two or more stents with a flexible polymeric tube. This technique may also be used to join a stent to another prosthetic device such as a tube, a graft and the like. Thus, in this embodiment of the invention, the stent is incorporated into an endoluminal prosthesis.
In yet another embodiment of the invention the stent may be secured (e.g. by suturing) to an existing endolurninal prosthesis such as Gortex(trademark) material or to biological material such as basilic vein. In this regard, securing of the stent to the existing endoluminal prosthesis or biological material may be facilitated by designing the stent such that an end of the stent comprises an annular row of the above-mentioned polygons is having a convex-shaped wall with a flat apex.